A problem with recently developed advanced CMOS (complementary MOS) devices with increasingly miniaturized transistors is degradation of a driving current caused by depletion of a gate electrode. Thus, to improve driving capability, efforts have been made to develop a technique using a metal-based material instead of polycrystalline silicon, which is conventionally used, that is, to develop what is called a metal gate technique.
On the other hand, the miniaturized transistor has resulted in a thinned gate insulating film, disadvantageously increasing a gate leakage current. Thus, in order to reduce power consumption, attempts have been made to use a high-dielectric-constant material (high-k material) for the gate insulating film to increase the physical film thickness thereof, thus reducing the gate leakage current.
Pure metal, metal nitride, a silicide material, or the like has been examined as a material for a metal gate electrode. In any case, (1) the gate insulating film needs to be prevented from being degraded when the metal gate electrode is formed, and (2) threshold voltages (Vth) for an N-type MOSFET and a P-type MOSFET need to be able to be set to proper values.
As means for achieving this, a method of controlling the Vth of the transistor has been proposed which method produces the gate electrodes of the N- and P-type MOSFETs using different metals or alloys with respective optimum work functions (this method is called a dual metal gate technique).
For example, Non-Patent Document 1 (International Electron Device Meeting Technical Digest 2002, p. 359) states that the work functions of a Ta electrode and a Ru electrode formed on SiO2 are 4.15 eV and 4.95 eV, respectively and that work function modulation of 0.8 eV can be performed between the two electrodes.
However, owing to the need to form metal layers made up of dissimilar metals or alloys with different work functions on a substrate, the dual metal gate technique involves a process of etching the metal layer deposited on the gate insulating film of one of the P- and N-type MOSFETs. Thus, disadvantageously, the etching degrades the quality of the gate insulating film, thus deteriorating the performance and reliability of elements.
On the other hand, Non-Patent Document 2 (International Electron Device Meeting Technical Digest 2004, p. 91) and Patent Document 1 (WO 2006/001271) describe a technique for an Ni full silicide gate electrode obtained by fully siliciding a gate pattern made up of polycrystalline silicon, with nickel (Ni). The technique carries out a high-temperature heat treatment to activate impurities in source/drain diffusion regions of a CMOS and then carries out a silicide process on a gate pattern made up of polycrystalline silicon for silicidation. Thus, the technique matches a conventional CMOS process well. The technique also enables gate electrodes made up of silicides with different work functions to be produced without the need for a process of etching away films deposited on the gate insulating film as in the case of the dual metal gate technique. Thus, possible damage to the gate insulating film can be prevented.
Specifically, Non-Patent Document 2 and Patent Document 1 describe that in production of a MOSFET including an HfSiON high-dielectric-constant film as a gate insulating film and an Ni full silicide electrode as a gate electrode, a wide range of effective work functions can be controlled by utilizing formation of a crystal phase to control the composition of nickel silicide. In particular, the documents describe that a Vth of ±0.3 V can be obtained by utilizing formation of an Ni3Si phase, an NiSi phase, and an NiSi2 phase.
In addition, as a technique for controlling the threshold value, a technique using a silicide containing impurities as a gate electrode has been proposed.
Patent Document 2 (US 2005/0070062-A) discloses a semiconductor device which uses a metal silicide with p-type impurities added thereto as a gate electrode for a P-type MOSFET and a metal silicide with n-type impurities added thereto as a gate electrode for an N-type MOSFET.
Patent Document 3 (JP 2005-129551-A) describes that desired work functions for a p-type MOSFET and an N-type MOSFET are obtained by using a gate insulating film made up of silicon oxide and a gate electrode containing 40 to 70 atom percents of Ni composition (Ni/[Ni+Si]) and p-type impurities, and a gate insulating film made up of silicon oxide and a gate electrode containing 30 to 60 atom percents of Ni composition and n-type impurities, respectively.
A technique of using an Ni full silicide gate electrode formed entirely of nickel silicide normally uses nickel monosilicide (NiSi) with a low resistance as a gate material. However, if a fine gate electrode and a relatively large gate electrode are simultaneously formed, silicidation disadvantageously progresses unevenly to form crystal phases with compositions varying depending on gate sizes. This is probably because the amount of nickel diffusing from a nickel film formed on a polycrystalline silicon layer making up a gate pattern to the polycrystalline silicon layer, particularly the amount of nickel diffusing in a lateral direction (a planar direction of a substrate), varies depending on the gate size. Furthermore, even when the gate electrodes have the same size, the speed of the silicidation is affected depending on the type of impurities added to the polycrystalline silicon layer. This prevents sufficient silicidation, with polycrystalline silicon remaining in the electrodes.
This phenomenon is probably because the NiSi in the nickel silicide is not in the most stable phase and because various crystal phases such as an NiSi2 phase, an Ni2Si phase, and an Ni3Si phase are likely to be formed. A possible variation in silicide composition varies the amount of impurities in the vicinity of an interface between the gate electrode and the gate insulating film. As a result, the threshold value varies. If the polycrystalline silicon remains in the electrodes without being silicided, original metal gate effects cannot be exerted.
To solve this problem, a technique of carrying out a two-level heat treatment for silicidation has been proposed. Such a technique is described in Non-Patent Document 3 (International Electron Devices Meeting Technical Digest, 2004, p. 87) and Non-Patent Document 4 (International Electron Devices Meeting Technical Digest, 2005, p. 661). For example, a first heat treatment is carried out to silicide polycrystalline silicon, and the silicidation is stopped before completion (so as to leave a polycrystalline silicon portion) to form a nickel-rich silicide such as Ni2Si. Then, surplus Ni is removed, and a second heat treatment is carried out to silicide the remaining polycrystalline silicon to form NiSi. This enables avoidance of lateral feeding of an excessive amount of Ni to fine patterns.
However, even with this technique, when miniaturization progresses to reduce the height particularly of the gate electrode, process margins decrease to make process control difficult. Then, the amount of Ni supplied to the polycrystalline silicon varies. In particular, formation of the nickel-rich silicide during the first heat treatment of the technique is likely to be affected by an insignificant temperature distribution on a wafer. Furthermore, the first heat treatment results in significant roughness reflecting grain boundaries at the interface between the silicide and the polycrystalline silicon. This makes control of the amount of Ni supplied to the polycrystalline silicon difficult.